1. Technical Field
The present disclosure relates to an ultrasonic surgical system. More particularly, but not exclusively, it relates to an ultrasonic surgical system able to achieve precise control of a desired operating point.
2. Background of Related Art
Devices which effectively utilize ultrasonic energy for a variety of applications are well-known in a number of diverse arts. A laparoscopic tool where the surgeon may use a scissors-type, a pistol or trigger type grip outside the body to operate a manipulative, gripping or clamping mechanism at a distal end of the tool within the body is useful for use with ultrasonically operated haemostatic cutting tools. Such haemostatic cutting tools are known from British Patent Number 2333709B, International Patent Applications Numbers PCT/GB99/00162 and PCT/GBOO/01580, and U.S. Pat. No. 5,322,055.
Each of the above identified patents and patent applications describes a surgical tool comprising means to generate ultrasonic vibrations and a waveguide, operatively connected at a proximal end to said generating means, and provided at a distal end with cutting and/or coagulating means. Each tool is provided with a jaw to hold tissue to be treated in contact with the ultrasonically vibrating cutting and/or coagulating means.
The Ampulla (Gaussian) profile was published by Kleesattel (as early as 1962), and is employed as a basis for many ultrasonic devices in surgical applications including devices patented and commercialized by Cavitron and Valleylab (patents by Wuchinich, et al., 1977, Stoddard, et al., 2001) for use in ultrasonic aspiration. The Gaussian profile is used in practice to establish and control the resonance and mechanical gain of devices. A resonator, a connecting body and the device act together as a three-body system to provide a mechanical gain, which is defined as the ratio of output stroke amplitude of the radiating tip to the input amplitude of the resonator. The mechanical gain is the result of the strain induced in the materials of which the resonator, the connecting body and the ultrasonic device are composed.
The magnetostrictive transducer coupled with the connecting body functions as the first stage of the booster device with a mechanical gain of about 2:1, due to the reduction in area ratio of the wall of the complex geometry. The major diameter of the device transitions to the large diameter of the Gaussian in a stepped device geometry with a gain of as large as about 5:1, again due to reduction in area ratio. The mechanical gain increases in the Gaussian due to the Square Root of (1+2*Ln (Area Ratio)), where Ln is the natural logarithm, or about 2:1 for the devices of interest. The total mechanical gain is the product of these constituents, or as large as 20:1 for this example. Thus, the application of ultrasonically vibrating surgical devices used to fragment and remove unwanted tissue with significant precision and safety has led to the development of a number of valuable surgical procedures. Accordingly, the use of ultrasonic aspirators for the fragmentation and surgical removal of tissue from a body has become known. Initially, the technique of surgical aspiration was applied for the fragmentation and removal of cataract tissue. Later, such techniques were applied with significant success to neurosurgery and other surgical specialties where the application of ultrasonic technology through a handheld device for selectively removing tissue on a layer-by-layer basis with precise control has proven feasible.
Certain devices known in the art characteristically produce continuous vibrations having substantially constant amplitude at a predetermined frequency (i.e. 20-30 kHz). Certain limitations have emerged in attempts to use such devices in a broad spectrum of surgical procedures. For example, the action of a continuously vibrating tip may not have a desired effect in breaking up certain types of body tissue, bone, etc. Because the ultrasonic frequency is limited by the physical characteristics of the handheld device, only the motion available at the tip provides the needed motion to break up a particular tissue. All interaction with the tissue is at the tip, some being purely mechanical and some being ultrasonic. The devices may have limitations in fragmenting some tissues. The limited focus of such a device may render it ineffective for certain applications due to the vibrations which may be provided by the handheld device. For certain medical procedures, it may be necessary to use multiple hand held devices or it may be necessary to use the same console for powering different handheld devices.
Certain devices known in the art characteristically produce continuous vibrations having a substantially constant amplitude at a frequency of about twenty to about thirty kHz up to about forty to about fifty kHz. The amplitude is inversely proportional to frequency and directly proportional to wavelength because the higher frequency transducers generally have less powerful resonators. For example, U.S. Pat. Nos. 4,063,557, 4,223,676 and 4,425,115 disclose devices suitable for the removal of soft tissue which are particularly adapted for removing highly compliant elastic tissue mixed with blood. Such devices are adapted to be continuously operated when the surgeon wishes to fragment and remove tissue.
A known instrument for the ultrasonic fragmentation of tissue at an operation site and aspiration of the tissue particles and fluid away from the site is the CUSA™ 200 System Ultrasonic Aspirator; see also U.S. Pat. No. 4,827,911, now sold as the CUSA Excel™. When the longitudinally vibrating tip in such an aspirator is brought into contact with tissue, it gently, selectively and precisely fragments and removes the tissue. Depending on the reserve power of the transducer, the CUSA transducer amplitude can be adjusted independently of the frequency. In simple harmonic motion devices, the frequency is independent of amplitude. Advantages of this unique surgical instrument include minimal damage to healthy tissue in a tumor removal procedure, skeletoning of blood vessels, prompt healing of tissue, minimal heating or tearing of margins of surrounding tissue, minimal pulling of healthy tissue, and excellent tactile feedback for selectively controlled tissue fragmentation and removal.
In many surgical procedures where ultrasonic fragmentation instruments are employed, additional instruments are required for tissue cutting and hemostasis at the operation site. For example, hemostasis is needed in desiccation techniques for deep coagulation to dry out large volumes of tissue and also in fulguration techniques for spray coagulation to dry out the surface of tissues.
The apparatus disclosed in U.S. Pat. Nos. 4,931,047 and 5,015,227 provide hemostasis in combination with an ultrasonically vibrating surgical fragmentation instrument and aspirator. The apparatus effectively provide both a coagulation capability and an enhanced ability to fragment and aspirate tissue in a manner which reduces trauma to surrounding tissue.
U.S. Pat. No. 4,750,488 and its two continuation Patents, U.S. Pat. Nos. 4,750,901 and 4,922,902, disclose methods and apparatus which utilize a combination of ultrasonic fragmentation, aspiration and cauterization.
In U.S. Pat. No. 5,462,522, there is disclosed, an ultrasonic therapeutic apparatus. The apparatus includes a water supply unit for supplying cooling water to cool the probe; a suction unit for removing waste matter by suction from the organic tissue treated by means of the cooling water and the probe; an ultrasonic output setting section for setting a preset value for an ultrasonic output from the ultrasonic vibrator; a feedwater output setting section for setting a preset value for a feedwater output from the water supply unit; and a feedwater output control section for controlling the feedwater output setting by the feedwater output setting section so that the preset feedwater output value is a value such that the probe is cooled and is not excessively heated.
In U.S. Published Application 2009/0143805 A1, there is disclosed, cutting instruments that utilize ultrasonic waves generate vibrations with an ultrasonic transducer along a longitudinal axis of a cutting blade. By placing a resonant wave along the length of the blade, high-speed longitudinal mechanical movement is produced at the end of the blade. These instruments are advantageous because the mechanical vibrations transmitted to the end of the blade are very effective at cutting organic tissue and, simultaneously, coagulate the tissue using the heat energy produced by the ultrasonic frequencies. Such instruments are particularly well suited for use in minimally invasive procedures, such as endoscopic or laparoscopic procedures, where the blade is passed through a trocar to reach the surgical site.
In an apparatus which fragments, cuts or coagulate tissue by the ultrasonic vibration of a tool tip, it is desirable, for optimum efficiency and energy utilization, that the transducer which provides the ultrasonic vibration operate at resonant frequency. The transducer design establishes the resonant frequency of the system, while the generator tracks the resonant frequency. The generator produces the electrical driving signal to vibrate the transducer at resonant frequency. However, changes in operational parameters, such as, changes in temperature, thermal expansion and load impedance, result in deviations in the resonant frequency.
More specifically, as the temperature increases, the material density decreases and the speed of sound increases. The increase in temperature may lead to a lower equivalent mass of the key system components, especially the device which has a very low mass and can heat up and cool down quickly. The lower equivalent mass may lead to a change in equivalent resonant frequency. Additionally, when the water supply unit supplies water to cool down the device, the water adds mass to the device as well as acting as a coolant to maintain the temperature of the device. As such, the presence of water may change the equivalent resonant frequency.